Evaluation of Reliability and Stack-Up Height on Next Generation 0.5 mm Pitch PoP

Abstract: As Package on Package (POP) technology becomes more prevalent and widely adopted across the semiconductor industry, there is a simultaneous push from the end user to make the entire stack-up thinner. The overall height of the tallest package mounted on the PCB, often the POP, becomes the major factor that dictates product design thickness. There are multiple variables in the POP package design to help address this challenge. This paper explores different configurations of the bottom POP component while holding the memory component constant.

The lower package is 14 mm in size, uses a 0.4 mm bottom pitch with 904 solderballs. The top package is also 14 mm, with a 0.5 mm pitch with 240 solderballs. The interconnect between the packages are joined through soldered connections embedded within vias of the bottom package’s mold compound. Three bottom solderball diameters were characterized as a drop-in solution to decreasing overall package thickness. Selected package configurations were evaluated based on drop shock, thermal cycling, and cyclic bend reliability test; focusing on both the bottom and top POP interface. Two mold compound via dimensions and two topball solder alloy compositions, Sn3.0Ag0.5Cu (SAC305) and Sn1.2Ag0.5Cu/0.05Ni (SAC125/Ni), were also studied for their effect on total collapsed height and reliability.

The intent of this study is to understand how different assembly variations affect the final assembled structure and board level reliability in order to determine which methods are realistic options for this next generation POP configuration. Package designs in this study were subject to 200 mechanical drops at 1500g and 0.5 ms, 1000 thermal cycles from -40°C to 125°C, and, when tested, 100,000 cyclic bends at 2 mm amplitude with a 1 Hz frequency. Coplanarity and high temperature warpage for the various configurations was also monitored.

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